Nowadays, electric power energy systems are important social infrastructure which cannot be stopped even for a moment. However, in an abnormality or a trouble of a load that causes an overcurrent, a measure taken thereagainst is a high-speed breaking of the load, as is exercised by a fuse or a high-speed mechanical switch. Nevertheless, there has been a demand for a high-function switch, so called a controller or a current limiter, which is capable of limiting only the overcurrent and allowing a continued operation without the complete stop of the load, as well as a system recovery to a full operation after the return to its normality.
Electric power system must be designed to withstand a short-time overcurrent, such as a rush current at the time an incandescent lamp is lit, a rush current when an induction motor is started, or an overcurrent caused by initial excitation inrush of a transformer. It is important to distribute yield strength of each machine appropriately. A semiconductor-type inverter power supply in recent years, such as a fuel battery inverter, for example, cannot withstand, in many cases, a peak current which is almost ten times the excitation inrush current of a transformer. Inverter power supplies, therefore, have various soft-start functions, which work if there is one load for one inverter power supply but have a difficulty in coping with later-started ones of a plurality of loads connected to one inverter power supply.
Electric power systems are designed in consideration of protective coordination, the current and the duration thereof to withstand an accidental, short-time overcurrent. However, such systems merely perform a protective coordination aimed at a prevention of the influence over the upstream by selectively breaking the accident current by a switch. It is a recent social demand to achieve a continuous operation as far as possible without power breaking in an accident that takes place in the downstream of a system.
As for a current limiter which limits an accident current with series elements, an application based on a transient phenomenon between superconductivity and normal conduction is developed. This is because the capacity of the breaker becomes extremely large as the accident current becomes excessively large, so that reduction in accident current to a half or so, if possible, can reduce the size and cost the breaker. In addition, such a current limiter is demanded.
There is a magnetic energy recovery switch (hereinafter called “MERS”) which can perform power control on a load. The MERS includes a full bridge circuit having four reverse-conductive type semiconductor switches, and a capacitor connected between the DC terminals of the full bridge circuit. The capacitor serves to store magnetic energy when the current is cut off, and recover the magnetic energy to the load. A gate control signal is sent to the gates of a pair of reverse-conductive type semiconductor switches positioned diagonally in the full bridge circuit. The current phase can be controlled by alternately turning on/off the two pairs of reverse-conductive type semiconductor switches so that one pair of reverse-conductive type semiconductor switches are turned on when the other pair of reverse-conductive type semiconductor switches are turned off by the gate control signal. Further, if the phase of the gate control signal is controlled in synchronization with the frequency of the AC power supply, the current phase can be controlled arbitrarily. When the load connected to the MERS is an inductive load, a voltage to the load can be increased or decreased by advancing the current phase. This has already registered as a patent and is disclosed (see Patent Document 1). The MERS in this mode is called “full-bridge type MERS”.
It has been applied, and laid open, and is publicly known that as the MERS's include simpler MERS circuits which can be constituted by two reverse-conductive type semiconductor switches though partly limited the functions of the full-bridge type MERS are (see Patent Document 2 and Patent Document 3).    Patent Document 1: Japanese Patent No. 3634982    Patent Document 2: Unexamined Japanese Patent Application KOKAI Publication No. 2007-58676    Patent Document 3: Unexamined Japanese Patent Application KOKAI Publication No. 2008-92745
Of the simple MERS circuits, a so-called vertical half-bridge type MERS is a half-bridged mode of a full bridge circuit. More specifically, of the four reverse-conductive type semiconductor switches connected in bridge in the full-bridge type MERS, two reverse-conductive type semiconductor switches which are connected to one AC terminal are replaced with diodes connected in the reverse directions. With a capacitor having the same capacity being added, the capacitors are respectively connected in parallel to the diodes.
Of the simple MERS circuits, a so-called horizontal half-bridge type MERS is also a half-bridged mode of a full bridge circuit, and differs from the vertical half-bridge type MERS in a half-bridging approach. In the horizontal half-bridge circuit a lower half of the full bridge circuit is used. The lower half means the part that is the lower one when the full bridge is separated laterally (horizontally) by the capacitor connected therein. Further, this horizontal half-bridge circuit has an additional capacitor with the same capacity. More specifically, two circuits each having a capacitor connected in parallel to the reverse-conductive type semiconductor switch are connected in series to each other, and the reverse-conductive type semiconductor switches are respectively connected in series to each other at the time of connecting the circuits in series.
Although the vertical half-bridge type MERS and the horizontal half-bridge type MERS need twice the quantity of capacitors in use as compared with the full-bridge type MERS, the quantity of reverse-conductive type semiconductor switches in user is reduced to a half. Therefore, the number of reverse-conductive type semiconductor switches through which the current passes is reduced and conduction loss is reduced. Because the current duty per a single capacitor (the amount of the current passing through a capacitor per unit time) is reduced to a half, the life of capacitors generally becomes longer. In addition, the basic electric characteristic relating to recovery of magnetic energy becomes substantially equivalent to that of the full-bridge type MERS. Both of the vertical half-bridge type MERS and horizontal half-bridge type MERS are advantageous over the full-bridge type MERS particularly at the time of application targeting a large current.